Field of the Invention
This invention relates to an oily inkjet ink adapted for use in inkjet recording systems.
Description of the Related Art
Inkjet recording systems are the systems in which an inkjet ink having a high degree of fluidity is jetted out in the form of ink droplets from fine head nozzles, and in which an image is recorded by the ink droplets on printing paper located so as to stand facing the nozzles. Recently, the inkjet recording systems have rapidly become popular for the possibility of high speed printing by use of line head type inkjet recording apparatuses provided with a plurality of ink heads. As the inks for use in the inkjet recording systems, there have been proposed various oily inkjet inks, which contain a water-insoluble solvent and a pigment finely dispersed in the water-insoluble solvent.
For example, in Japanese Unexamined Patent Publication No. 2007-126564, the inventors proposed an ink, which contains a non-polar solvent, such as an ester solvent, a higher alcohol solvent, or a hydrocarbon solvent, and a pigment dispersed in the non-polar solvent. The proposed ink has the advantages in that the ink has excellent on-printer stability, and in that the ink yields a printed surface, which does not adhere to a print surface having been printed with a plain paper copier or a laser printer when being superimposed upon the print surface.
The oily inkjet ink is not an ink which dries and solidifies by itself, but is a penetration drying type ink which penetrates through a printed material, such as paper, and thereby dries. Therefore, in cases where high speed printing is performed, the period of time elapsing from the printing to the outputting becomes short, the problems with regard to the so-called “roller transfer staining” occur in that the ink having been printed on a paper surface and having not been dried is transferred to a conveying roller and then from the conveying roller to printed paper conveyed next, and thus stains the printed paper.
As a technique for improving fixing characteristics of an oily inkjet ink, Japanese Unexamined Patent Publication No. 2003-268266 describes that the fixing characteristics of the oily inkjet ink can be improved by adjusting the number of an ester group, the number of an ether group, and the carbon number of an alkyl group in a solvent. Also, as an oily inkjet ink having good penetrability on printed paper and having good fixing characteristics of the ink immediately after printing, Japanese Unexamined Patent Publication No. 2004-137315 proposes an ink which contains a hydrocarbon type solvent as a principal solvent and which contains an ester derivative of a (poly)alkylene glycol. Further, as an ink having good printing quality, Japanese Unexamined Patent Publication No. 2007-161890 proposes an oily inkjet ink containing a fatty acid ester and a non-aqueous polar solvent as solvents.
With the ink described in Japanese Unexamined Patent Publication No. 2004-137315, though an improvement in ink penetration drying characteristics is obtained, storage stability and jetting-out stability become bad due to low solubility between the solvents. The ink described in Japanese Unexamined Patent Publication No. 2007-161890 has the problems in that, in cases where the ink is used for printing on plain paper or reprocessed paper, the penetration drying characteristics become bad. The penetration drying characteristics of an oily inkjet ink depend markedly upon ink viscosity. However, the ink heads of the inkjet recording systems impose marked limitation of the ink viscosity, and therefore the viscosity adjustment is possible only within a viscosity range in which the ink can be jetted out. If the viscosity range in which the ink can be jetted out is ignored, the jetting-out stability and the storage stability will become bad. Therefore, it is not always possible to improve and enhance the penetration drying characteristics easily as described in Japanese Unexamined Patent Publication No. 2003-268266.
As described above, the penetration drying characteristics of the ink depend markedly upon the viscosity and affect the jetting-out stability and the storage stability. Therefore, the inventors eagerly studied with regard to whether or not the penetration drying characteristics can be controlled by constituent factors other than the viscosity, and found that, by the containing of solvents varying in polarity in the ink, the penetration drying characteristics are enhanced, and the roller transfer staining is suppressed. The present invention is based upon the findings described above.